1. Field of the Invention
The present invention relates to a method of producing a pattern for molding castings, and more particularly to a method of producing a pattern used for molding, through the full mold process, a pressing die for pressing an automotive body panel or the like.
2. Description of the Relevant Art
FIG. 9 of the accompanying drawings shows a conventional press die assembly which has upper and lower pressing dies 100a, 100b and blank holders 101a, 101b disposed around the upper and lower pressing dies 100a, 100b, respectively. The blank holder 101b is supported on cushion pins 103 mounted on a press frame 102 and grips the peripheral edge of a steel sheet blank (not shown) to prevent the steel sheet blank from developing wrinkles. The lower pressing die 100b has a shaping panel 104 for being pressed against the lower surface of the steel sheet blank, an attachment panel 105 fixedly mounted on the press frame 102, and ribs 106 interconnecting the panels 104, 105.
One known method of designing such a pressing die is disclosed in Japanese Laid-Open Patent Publication No. 62-182806 published Aug. 11, 1987. According to the disclosed method, when certain conditional data on a pressing die which is to be produced are entered, then other configurations of the pressing die are automatically determined. For example, when the type and position of bolts to be attached to die plates are specified, the configuration of the bolts and the machined shape of the die plates are automatically displayed on a CRT, and NC (numerical control) data for machining the die plates are automatically prepared.
Japanese Laid-Open Patent Publication No. 62-14208 published Jan. 22, 1987 also discloses an automatic NC data generating system, which is however not specifically addressed to the design of dies. According to the disclosed system, NC data such as a laser output, a machining speed, a pulse condition, etc. are automatically generated simply by selecting certain required data on a product to be laser-machined, such as the material and thickness of the product, the accuracy with which the product is to be machined, etc.
The full mold process has heretofore been employed to produce a molded die regardless of whether the die is designed by CAD (computer-aided design) or a manual designing process. In the full mold process, a pattern having the same shape as the shape of a die to be produced is prepared by cutting a plate-like blank made of a material such as foamed styrene into parts, assembling them into one pattern blank, and forming curved surfaces on the pattern blank by profiling. The produced pattern is then embedded in resin sand whose sand particles are coated with resin. Molten metal is then poured into the pattern, which is eliminated and replaced with a die having the same shape as that of the pattern.
The ribs 106 (FIG. 9) should best be positioned such that they will most effectively bear the pressing force which is applied from the upper die 100a through the steel sheet blank to the shaping panel 104 of the lower die 100b. More specifically, the outer rib 106 should extend downwardly from the outermost edge of the shaping panel 104 as indicated by the imaginary lines 106'.
If the outer rib were positioned so as to extend downwardly from the outermost edge of the shaping panel 104, however, the rib would interfere with the cushion pins 103 which are positionally inherent in the press frame 102. Therefore, the ribs must be disposed between the cushion pins 103, and are not optimally positioned. In order for the ribs 106 to withstand the applied pressing force, the ribs 106 should be large in wall thickness, and hence the ribs 106 are not of optimum shape and the die 100b has a large weight.
The pattern to be used in the full mold process has a surface corresponding to the reverse side of the shaping panel 104, and such a surface cannot easily be finished by machining. The molded die is required to be of a wall thickness large enough to give sufficient mechanical strength to the molded die. The surface of the pattern corresponding to the reverse side of the shaping panel 104 is irregular or rough as it is not machined, and hence the thickness of the shaping panel 104 is irregular. This also results in an increase in the weight of the die 100b.
To produce those parts of the pattern which cannot be machined, a plate-like blank having the same thickness as that of a rib, for example, is marked to actual parts dimensions and then cut into components, e.g., several hundred components in the case of the die 100b, and the components are then manually bonded together. This manual process requires much skill and many production steps, is highly costly due to required manual labor, and requires a long period of time to complete one pattern.
The present invention has been made in an effort to effectively solve the aforesaid problems of the conventional methods of producing patterns for molding castings.